A Snellen chart is an eye chart that can be used to measure visual acuity. Snellen charts are named after the Dutch ophthalmologist Herman Snellen who developed the chart in 1862 as a measurement tool for the acuity formula developed by his professor Franciscus Cornelis Donders.[1][2] Many ophthalmologists and vision scientists now use an improved chart known as the LogMAR chart.
Snellen developed charts using symbols based in a 55 unit grid. The experimental charts developed in 1861 used abstract symbols.[3] Snellen's charts published in 1862 used alphanumeric capitals in the 55 grid. The original chart shows A, C, E, G, L, N, P, R, T, 5, V, Z, B, D, 4, F, H, K, O, S, 3, U, Y, A, C, E, G, L, 2.[2]
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The normal Snellen chart is printed with eleven lines of block letters. The first lineconsists of one very large letter, which may be one of several letters, for example E, H, or N. Subsequent rows have increasing numbers of letters that decrease in size. A person taking the test covers one eye from 6 metres or 20 feet away, and reads aloud the letters of each row, beginning at the top. The smallest row that can be read accurately indicates the visual acuity in that specific eye.The symbols on an acuity chart are formally known as "optotypes".
In the case of the traditional Snellen chart, the optotypes have the appearance of block letters, and are intended to be seen and read as letters. They are not, however, letters from any ordinary typographer's font. They have a particular, simple geometry in which:
Outside the United States, the standard chart distance is 6 metres (20 ft), and normal acuity is designated "6/6". Other acuities are expressed as ratios with a numerator of 6. Some clinics do not have 6-metre eye lanes available, and either a half-size chart subtending the same angles at 3 metres (9.8 ft), or a reversed chart projected and viewed by a mirror is used to achieve the correct sized letters.
In the most familiar acuity test, a Snellen chart is placed at a standard distance: 6 metres. At this distance, the symbols on the line representing "normal" acuity subtend an angle of five minutes of arc, and the thickness of the lines and of the spaces between the lines subtends one minute of arc. This line, designated 6/6 (or 20/20), is the smallest line that a person with normal acuity can read at a distance of 6 metres. This definition is arbitrary, since human eyes typically have higher acuity, as Tscherning writes, "We have found also that the best eyes have a visual acuity which approaches 2, and we can be almost certain that if, with a good illumination, the acuity is only equal to 1, the eye presents defects sufficiently pronounced to be easily established."[6]
At exactly 6 metres' distance from the patient, the letters on the 6/6 line shall subtend 5 minutes of arc (such that the individual limbs of the letters subtend 1 minute of arc), which means that the chart should be sized such that these letters are 8.73 mm tall and the topmost (6/60) "E" should be 87.3 mm tall. Putting it another way, the eye should be at a distance 68.76 times the height of the top (6/60) letter. The formula is
where w {\displaystyle w} is the optotype height or width (which are the same due to the optotype being on a square grid), d {\displaystyle d} is the distance from eye to chart, and {\displaystyle \theta } is the angle subtended by the optotype, which is 5 arcminutes as specified by Snellen.[7] Another calculation for United States clinics using 20-foot chart distances (slightly more than 6 m), and using a 17 mm model eye for calculations, and a letter which subtends 5 minutes of arc, gives a vertical height of the 20/20 letter to be 8.75 mm.[8]
The largest letter on an eye chart often represents an acuity of 6/60 (20/200), the value that is considered "legally blind" in the US. Many individuals with high myopia cannot read the large E without glasses, but can read the 6/6 (20/20) line or 6/4.5 (20/15) line with glasses. By contrast, legally blind individuals have a visual acuity of 6/60 (20/200) or less when using the best corrective lens.
Since computer monitors typically have good lighting for reading and LCD/LED monitors have high DPI (between 96 and 480) they are suitable for displaying optotypes. Commonly digital chart products support randomizing optotypes displayed to prevent patients from memorizing lines they have previously read. In Google Play and App Store (iOS), there are Snellen chart apps for smart phones and tablets.
Invented in 1862 by a Dutch ophthalmologist named Herman Snellen, the Snellen chart remains the most widespread technique in clinical practice for measuring visual acuity.[1][2] The Snellen chart serves as a portable tool to quickly assess monocular and binocular visual acuity. Throughout history, there have been several charts utilized by eye care professionals. The most common in clinical practice is the Snellen chart, while research studies more often use logMAR charts, such as the Early Treatment of Diabetic Retinopathy Study (ETDRS) chart.
The Snellen Chart uses a geometric scale to measure visual acuity, with normal vision at a distance being set at 20/20. The numerator represents the distance that the patient is standing from the chart (in feet), while the denominator represents the distance from which a person with perfect eyesight is still able to read the smallest line that the patient can clearly visualize. For example, a patient standing 20 feet away from the chart who can clearly read until the line of font that a person with normal visual acuity can read from 40 feet away would be measured as 20/40 vision. A similar assessment for testing near vision can be done using a pocket card held about 14 inches from the patient's eyes. There are only nine letters on the chart, known as optotypes: C, D, E, F, L, O, P, T, and Z. Finally, the sizing of letters is geometrically consistent, meaning that optotypes representing 20/40 are twice the size of those representing 20/20.
While the Snellen chart is prized as a quick, low cost, and portable tool, there are several drawbacks to its use. These disadvantages, outlined below, gave birth to the improvements seen in newer charts, such as the ETDRS.
Every line of the chart contains a different number of characters, i.e., eight characters on the 20/20 line and only 1 letter on the 20/200 line.[8] This presents confusion when a patient's medical record shows, for example, a gain of 3 letters of acuity, which could indicate a small improvement on the 20/20 line or a gain of an entire acuity line in the 20/70 acuity. Furthermore, the progression between lines is variable, with different spacing from line to line. As such, a two-line improvement in acuity could represent a gain of 33% from 20/30 to 20/20 or a gain of 50% from 20/100 to 20/50. The lack of standardization in line progression, as well as the spread of the measurements, makes it difficult to make statistical comparisons on acuity data.[9] Additionally, the distance between rows and characters varies considerably, creating a crowding phenomenon of adjacent contour interactions, reducing acuity.[10] Finally, some letters (i.e., L, T, A) are reportedly easier to see than other letters (i.e., B, S, C).[11]
Administering the Snellen chart test requires that the patient focus his/her attention for the duration of the test and cooperate by following the instructions given. For those patients who are unable to cooperate due a physical or mental disability or for pediatric patients, this test may prove challenging to administer.
An important component of the cranial nerve 2 (optic nerve) exam is testing the visual acuity. Visual acuity assessment has been used clinically for hundreds of years as a useful tool to gauge the clarity of a patient's vision. When interpreting visual acuity using the Snellen chart, it is important to note that 20/20 vision is not synonymous with perfect vision, but rather, indicates a measure of a patient's clarity of eyesight from a distance. Overall visual ability encompasses a variety of other vision skills, including peripheral vision, eye coordination, depth perception, ability to focus on objects, and color vision. Furthermore, visual acuity assessment using the Snellen chart does not give a patient's prescription. The prescription is a separate measurement that must be evaluated by an eye professional in an office setting.
An interprofessional care team of nurses, allied health professionals, and physicians must maintain a collaborative and communicative environment to achieve positive patient outcomes. Regarding the Snellen chart, the interprofessional team often includes primary care physicians, hospitalists, ophthalmologists, optometrists, technicians, and nurses. Standardized testing procedures, as outlined above, as well as consistent reporting of visual acuity measurements, provide greater reliability of Snellen chart test results. An example of this can be seen in the case of a hospitalist testing the visual acuity of a temporal arteritis patient in dim lighting and reporting the acuity as 20/70. In contrast, the next day, the ophthalmologist uses the Snellen chart in good lighting and measures the patient's acuity to be 20/50. Despite the progression of vision loss overnight, the Snellen chart results misleadingly suggest an improvement in visual acuity because of the lack of standardization in testing procedures.
Monitoring the attitudes, education, and readiness of interprofessional team members to use and interpret the Snellen chart can be vital for enhancing team dynamics. Kim et al. demonstrated that a single interprofessional education session for health science students, with Snellen chart training, improved the interprofessional attitudes, knowledge of the roles of other professions, and the perceived-readiness to work together.[15] [Level 3]
Various types of eye charts are available. Some charts use pictures or patterns, while others use letters. Eye care providers might use certain charts for measuring distance vision and others for measuring near vision. Some eye charts are especially for children while others work for both children and adults. The Snellen eye chart, however, is the most common and the most recognizable. ff782bc1db